Mount Vernon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Vernon clear sky chart from 2005-11-06 to 2024-11-02.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	21% 9% 3 7% 4 4 4 3 5% 6% 34%	101696
	Night	25% 10% 3 7% 4 4 4 3 4 5% 31%	97341
Transparency	Day	54% 5% 13% 18% 8% 2	101449
	Night	57% 5% 12% 17% 7% 2	97234
Seeing	Day	24% 10% 20% 26% 17% 4	15193
	Night	30% 9% 14% 25% 19% 3	23472

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 16% 4 7% 4 5% 4 2 4 4 12%	2219
	11th to 20th	Day	27% 14% 3 8% 5% 4 4 3 4 6% 23%	2368
	21st to end	Day	41% 14% 4 8% 5% 4 4 3 4 4 10%	2827
February	1st to 10th	Day	32% 13% 3 8% 5% 4 4 3 5% 6% 17%	1710
	11th to 20th	Day	29% 12% 5% 9% 5% 4 3 4 5% 5% 20%	1665
	21st to end	Day	28% 16% 3 8% 5% 4 5% 4 5% 5% 18%	1464
March	1st to 10th	Day	20% 16% 4 9% 6% 5% 5% 3 5% 7% 18%	3200
	11th to 20th	Day	20% 10% 3 7% 4 5% 4 3 5% 4 35%	3490
	21st to end	Day	26% 12% 4 9% 5% 5% 5% 4 5% 6% 18%	3963
April	1st to 10th	Day	25% 13% 4 8% 7% 5% 4 4 6% 6% 17%	2280
	11th to 20th	Day	22% 12% 3 8% 6% 6% 5% 4 5% 6% 24%	2294
	21st to end	Day	25% 11% 4 8% 6% 5% 5% 3 6% 6% 21%	2532
May	1st to 10th	Day	19% 10% 3 6% 5% 4 4 3 5% 6% 37%	4420
	11th to 20th	Day	15% 7% 3 6% 4 4 4 5% 6% 9% 38%	4377
	21st to end	Day	28% 10% 3 8% 5% 5% 4 4 5% 7% 22%	4813
June	1st to 10th	Day	23% 8% 3 7% 5% 4 4 4 6% 7% 30%	2787
	11th to 20th	Day	24% 10% 4 8% 5% 5% 5% 4 6% 6% 24%	2850
	21st to end	Day	14% 7% 2 6% 4 5% 5% 4 7% 7% 38%	2850
July	1st to 10th	Day	7% 4 2 3 2 3 3 2 3 5% 65%	4740
	11th to 20th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	4654
	21st to end	Day	12% 5% 2 5% 3 3 3 3 4 6% 55%	4790
August	1st to 10th	Day	6% 3 2 4 2 3 3 2 5% 6% 64%	4376
	11th to 20th	Day	10% 6% 2 7% 5% 4 4 4 6% 8% 44%	4026
	21st to end	Day	17% 7% 3 6% 3 4 3 3 4 7% 42%	4350
September	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 49%	2280
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 7% 40%	2280
	21st to end	Day	20% 8% 2 6% 3 4 4 3 5% 7% 37%	2009
October	1st to 10th	Day	22% 8% 2 7% 4 4 4 4 6% 7% 32%	1900
	11th to 20th	Day	27% 9% 3 8% 4 4 4 4 5% 7% 26%	1900
	21st to end	Day	31% 9% 4 8% 4 4 4 4 5% 5% 23%	1960
November	1st to 10th	Day	36% 10% 3 8% 4 4 4 3 5% 4 18%	1513
	11th to 20th	Day	34% 10% 3 7% 5% 5% 4 3 4 5% 21%	1520
	21st to end	Day	35% 10% 3 8% 4 4 4 3 4 6% 19%	1520
December	1st to 10th	Day	28% 14% 3 8% 5% 4 3 3 4 4 24%	1520
	11th to 20th	Day	32% 14% 3 10% 4 3 3 4 4 4 18%	1807
	21st to end	Day	29% 16% 5% 10% 6% 5% 4 4 4 5% 13%	2442
January	1st to 10th	Night	37% 15% 4 9% 5% 4 4 3 3 3 12%	4726
	11th to 20th	Night	30% 12% 3 7% 3 3 4 3 4 6% 24%	4527
	21st to end	Night	40% 13% 3 8% 5% 3 3 3 4 4 14%	4827
February	1st to 10th	Night	33% 13% 3 6% 4 4 3 3 4 5% 20%	2470
	11th to 20th	Night	30% 14% 3 7% 5% 4 4 3 4 5% 23%	2402
	21st to end	Night	31% 13% 3 7% 4 4 4 2 5% 5% 21%	1989
March	1st to 10th	Night	23% 14% 4 9% 5% 4 4 4 6% 5% 20%	3659
	11th to 20th	Night	23% 11% 2 5% 3 4 3 3 4 6% 36%	3489
	21st to end	Night	30% 14% 3 10% 5% 5% 4 3 4 4 16%	3689
April	1st to 10th	Night	27% 14% 3 8% 5% 5% 4 5% 5% 6% 18%	1900
	11th to 20th	Night	23% 11% 3 7% 6% 5% 5% 3 4 5% 27%	1833
	21st to end	Night	29% 11% 3 9% 5% 4 3 3 5% 6% 22%	1647
May	1st to 10th	Night	19% 8% 3 6% 3 3 3 2 4 5% 44%	2540
	11th to 20th	Night	13% 8% 2 7% 4 4 4 4 5% 7% 42%	2516
	21st to end	Night	25% 10% 4 9% 5% 4 5% 4 6% 5% 23%	2762
June	1st to 10th	Night	24% 7% 2 7% 6% 5% 4 3 5% 5% 33%	1389
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 5% 4 7% 29%	1330
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 7% 42%	1330
July	1st to 10th	Night	7% 4 2 3 2 3 3 2 3 5% 66%	2220
	11th to 20th	Night	6% 3 1 3 2 1 2 1 4 4 74%	2306
	21st to end	Night	12% 4 1 4 2 3 3 2 4 5% 59%	2762
August	1st to 10th	Night	5% 2 1 2 1 2 3 3 5% 5% 70%	2552
	11th to 20th	Night	11% 6% 2 6% 3 3 3 3 5% 6% 52%	2812
	21st to end	Night	16% 6% 2 6% 3 4 3 3 5% 7% 46%	3322
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 4 6% 55%	1900
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 4 6% 42%	1900
	21st to end	Night	19% 8% 2 5% 3 4 4 3 5% 6% 41%	2170
October	1st to 10th	Night	20% 7% 3 7% 5% 4 4 3 4 5% 38%	2280
	11th to 20th	Night	28% 9% 2 6% 4 3 4 3 4 5% 32%	2280
	21st to end	Night	28% 9% 3 7% 4 4 4 3 4 6% 28%	2637
November	1st to 10th	Night	38% 10% 3 7% 5% 4 3 3 4 5% 19%	2597
	11th to 20th	Night	36% 10% 3 6% 4 3 4 3 4 4 22%	2660
	21st to end	Night	35% 11% 3 6% 4 4 3 4 5% 5% 20%	2660
December	1st to 10th	Night	30% 11% 2 6% 4 4 3 3 4 5% 26%	2660
	11th to 20th	Night	33% 14% 4 9% 5% 4 4 3 4 5% 16%	3384
	21st to end	Night	32% 15% 4 9% 6% 5% 4 3 4 4 13%	5214

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	79% 5% 6% 7% 2 1	2219
	11th to 20th	Day	64% 5% 9% 13% 7% 3	2391
	21st to end	Day	80% 3 7% 7% 3	2827
February	1st to 10th	Day	71% 5% 8% 9% 5% 2	1710
	11th to 20th	Day	71% 3 8% 10% 7% 1	1674
	21st to end	Day	70% 4 8% 13% 3 1	1464
March	1st to 10th	Day	66% 7% 10% 13% 5%	3200
	11th to 20th	Day	55% 4 15% 23% 2 1	3490
	21st to end	Day	68% 7% 13% 11% 1	3963
April	1st to 10th	Day	70% 6% 14% 9% 1	2280
	11th to 20th	Day	65% 4 10% 13% 7% 2	2294
	21st to end	Day	66% 6% 10% 13% 5% 1	2532
May	1st to 10th	Day	52% 6% 10% 23% 9%	4420
	11th to 20th	Day	45% 8% 24% 19% 4	4380
	21st to end	Day	65% 7% 17% 10% 1	4814
June	1st to 10th	Day	57% 5% 18% 13% 6% 1	2787
	11th to 20th	Day	64% 5% 16% 12% 4	2850
	21st to end	Day	47% 6% 18% 20% 7% 1	2850
July	1st to 10th	Day	26% 4 11% 23% 22% 14%	4666
	11th to 20th	Day	21% 3 11% 34% 29% 3	4573
	21st to end	Day	33% 3 20% 31% 12% 2	4639
August	1st to 10th	Day	23% 4 16% 40% 17% 1	4359
	11th to 20th	Day	40% 3 25% 22% 7% 2	4065
	21st to end	Day	45% 3 14% 25% 11%	4349
September	1st to 10th	Day	37% 5% 16% 23% 13% 5%	2280
	11th to 20th	Day	46% 5% 14% 18% 15% 2	2280
	21st to end	Day	49% 5% 16% 18% 10% 2	2009
October	1st to 10th	Day	52% 4 16% 20% 7%	1900
	11th to 20th	Day	59% 5% 12% 18% 5% 1	1900
	21st to end	Day	64% 4 9% 15% 8%	1960
November	1st to 10th	Day	71% 5% 10% 9% 4 1	1513
	11th to 20th	Day	70% 4 8% 10% 6% 2	1520
	21st to end	Day	70% 4 5% 11% 9% 1	1520
December	1st to 10th	Day	65% 5% 9% 11% 9% 1	1520
	11th to 20th	Day	70% 5% 9% 12% 4 1	1809
	21st to end	Day	76% 6% 8% 8% 2	2442
January	1st to 10th	Night	79% 5% 5% 8% 2 1	4726
	11th to 20th	Night	61% 6% 8% 15% 8% 2	4566
	21st to end	Night	76% 4 7% 9% 3	4827
February	1st to 10th	Night	70% 4 9% 10% 4 2	2470
	11th to 20th	Night	67% 4 9% 11% 7% 2	2404
	21st to end	Night	68% 3 8% 15% 5% 1	1989
March	1st to 10th	Night	65% 5% 11% 12% 7%	3659
	11th to 20th	Night	51% 5% 16% 24% 3 1	3490
	21st to end	Night	71% 6% 12% 10% 1	3689
April	1st to 10th	Night	68% 7% 12% 12% 2	1900
	11th to 20th	Night	62% 4 13% 12% 7% 2	1833
	21st to end	Night	65% 5% 11% 12% 6% 1	1647
May	1st to 10th	Night	46% 4 13% 28% 8%	2540
	11th to 20th	Night	42% 8% 25% 20% 5%	2516
	21st to end	Night	61% 9% 18% 11% 1	2762
June	1st to 10th	Night	55% 4 18% 17% 5% 1	1389
	11th to 20th	Night	56% 6% 18% 16% 3	1330
	21st to end	Night	42% 6% 22% 23% 4 3	1330
July	1st to 10th	Night	24% 4 9% 25% 21% 18%	2185
	11th to 20th	Night	18% 3 11% 29% 37% 2	2264
	21st to end	Night	29% 4 14% 30% 20% 3	2674
August	1st to 10th	Night	18% 4 16% 42% 19% 2	2544
	11th to 20th	Night	32% 3 29% 25% 9% 2	2839
	21st to end	Night	41% 4 18% 27% 10% 1	3322
September	1st to 10th	Night	33% 4 16% 28% 13% 5%	1900
	11th to 20th	Night	46% 5% 12% 18% 15% 4	1900
	21st to end	Night	46% 6% 16% 21% 10% 2	2170
October	1st to 10th	Night	50% 5% 14% 23% 8% 1	2280
	11th to 20th	Night	57% 3 14% 17% 7% 1	2280
	21st to end	Night	61% 4 11% 16% 8%	2637
November	1st to 10th	Night	71% 3 10% 11% 4 1	2597
	11th to 20th	Night	68% 4 9% 10% 6% 2	2660
	21st to end	Night	70% 4 7% 9% 8% 1	2660
December	1st to 10th	Night	62% 6% 11% 13% 8% 1	2660
	11th to 20th	Night	72% 6% 9% 9% 4 1	3381
	21st to end	Night	75% 6% 8% 8% 3 1	5214

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	39% 16% 14% 15% 10% 6%	180
	11th to 20th	Day	33% 16% 21% 12% 13% 4	180
	21st to end	Day	40% 14% 17% 14% 14% 1	218
February	1st to 10th	Day	41% 10% 21% 18% 8% 1	370
	11th to 20th	Day	38% 13% 19% 20% 10% 1	365
	21st to end	Day	34% 14% 21% 20% 10% 1	332
March	1st to 10th	Day	31% 15% 24% 19% 9% 1	460
	11th to 20th	Day	30% 12% 19% 18% 17% 3	460
	21st to end	Day	34% 14% 20% 22% 9% 1	506
April	1st to 10th	Day	28% 15% 24% 20% 10% 3	460
	11th to 20th	Day	23% 15% 24% 23% 11% 3	452
	21st to end	Day	27% 14% 25% 20% 13% 2	460
May	1st to 10th	Day	16% 11% 25% 32% 14% 2	463
	11th to 20th	Day	24% 7% 18% 28% 17% 5%	488
	21st to end	Day	23% 11% 23% 25% 13% 5%	529
June	1st to 10th	Day	22% 9% 24% 28% 16% 2	617
	11th to 20th	Day	22% 10% 25% 30% 11% 2	680
	21st to end	Day	13% 7% 22% 31% 23% 4	680
July	1st to 10th	Day	12% 3 21% 36% 24% 4	671
	11th to 20th	Day	9% 7% 18% 38% 22% 6%	621
	21st to end	Day	8% 4 17% 34% 30% 7%	531
August	1st to 10th	Day	11% 4 12% 33% 30% 10%	487
	11th to 20th	Day	7% 3 16% 32% 32% 10%	477
	21st to end	Day	11% 6% 21% 33% 21% 7%	539
September	1st to 10th	Day	17% 5% 20% 33% 20% 5%	490
	11th to 20th	Day	17% 7% 18% 30% 23% 4	490
	21st to end	Day	25% 6% 21% 25% 18% 6%	490
October	1st to 10th	Day	25% 7% 19% 25% 17% 7%	490
	11th to 20th	Day	33% 10% 16% 21% 15% 5%	490
	21st to end	Day	30% 13% 20% 20% 13% 3	409
November	1st to 10th	Day	36% 15% 21% 17% 11% 1	190
	11th to 20th	Day	38% 12% 16% 13% 13% 7%	180
	21st to end	Day	34% 14% 21% 14% 9% 9%	180
December	1st to 10th	Day	45% 14% 8% 11% 13% 9%	180
	11th to 20th	Day	42% 12% 18% 15% 12% 1	180
	21st to end	Day	40% 12% 17% 25% 5% 2	198
January	1st to 10th	Night	43% 13% 17% 15% 10% 2	850
	11th to 20th	Night	40% 11% 14% 19% 14% 2	799
	21st to end	Night	40% 8% 18% 21% 11% 1	836
February	1st to 10th	Night	38% 10% 17% 23% 10% 2	760
	11th to 20th	Night	35% 10% 17% 25% 11% 1	744
	21st to end	Night	37% 9% 18% 23% 11% 2	628
March	1st to 10th	Night	34% 13% 17% 22% 12% 1	760
	11th to 20th	Night	35% 12% 16% 23% 13% 1	760
	21st to end	Night	37% 12% 18% 23% 9% 1	836
April	1st to 10th	Night	34% 10% 15% 25% 14% 3	760
	11th to 20th	Night	27% 11% 20% 25% 16% 2	705
	21st to end	Night	32% 8% 14% 32% 12% 2	507
May	1st to 10th	Night	20% 7% 12% 35% 23% 2	380
	11th to 20th	Night	22% 6% 11% 29% 27% 5%	374
	21st to end	Night	23% 7% 12% 26% 28% 4	411
June	1st to 10th	Night	25% 6% 12% 34% 20% 3	380
	11th to 20th	Night	23% 7% 10% 35% 21% 3	380
	21st to end	Night	17% 4 10% 32% 31% 5%	380
July	1st to 10th	Night	12% 4 6% 34% 40% 5%	370
	11th to 20th	Night	10% 5% 9% 33% 38% 6%	368
	21st to end	Night	10% 1 5% 32% 44% 8%	416
August	1st to 10th	Night	10% 2 4 23% 50% 11%	388
	11th to 20th	Night	10% 2 4 22% 49% 14%	564
	21st to end	Night	14% 4 8% 31% 37% 7%	725
September	1st to 10th	Night	13% 4 7% 37% 34% 6%	760
	11th to 20th	Night	19% 5% 9% 24% 32% 11%	760
	21st to end	Night	22% 5% 13% 27% 27% 6%	760
October	1st to 10th	Night	22% 6% 13% 27% 26% 5%	760
	11th to 20th	Night	33% 6% 14% 22% 20% 5%	760
	21st to end	Night	32% 10% 17% 25% 12% 3	836
November	1st to 10th	Night	40% 11% 18% 22% 9%	748
	11th to 20th	Night	38% 13% 18% 21% 9% 2	760
	21st to end	Night	40% 9% 18% 18% 11% 4	760
December	1st to 10th	Night	34% 8% 15% 25% 15% 2	760
	11th to 20th	Night	41% 12% 21% 19% 6% 1	792
	21st to end	Night	41% 13% 21% 18% 7%	935

Page generated at 2024-10-30 10:19:21. Version ac07

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